EDP Sciences
Free Access
Volume 390, Number 2, August I 2002
Page(s) 407 - 421
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361:20020662
Published online 12 July 2002

A&A 390, 407-421 (2002)
DOI: 10.1051/0004-6361:20020662

The WEBT BL Lacertae Campaign 2000

M. Villata1, C. M. Raiteri1, O. M. Kurtanidze2, 3, 4, M. G. Nikolashvili2, M. A. Ibrahimov5, 6, I. E. Papadakis7, 8, K. Tsinganos7, K. Sadakane9, N. Okada9, L. O. Takalo10, A. Sillanpää10, G. Tosti11, S. Ciprini11, A. Frasca12, E. Marilli12, R. M. Robb13, J. C. Noble14, S. G. Jorstad14, 15, V. A. Hagen-Thorn15, 16, V. M. Larionov15, 16, R. Nesci17, M. Maesano17, R. D. Schwartz18, J. Basler18, P. W. Gorham19, H. Iwamatsu20, T. Kato20, C. Pullen21, E. Benítez22, J. A. de Diego22, M. Moilanen23, A. Oksanen23, D. Rodriguez24, A. C. Sadun25, M. Kelly25, M. T. Carini26, H. R. Miller27, S. Catalano12, D. Dultzin-Hacyan22, J. H. Fan28, R. Ishioka20, H. Karttunen10, P. Keinänen10, N. A. Kudryavtseva15, M. Lainela10, L. Lanteri1, E. G. Larionova15, K. Matsumoto20, J. R. Mattox29, F. Montagni17, G. Nucciarelli11, L. Ostorero30, J. Papamastorakis7, 8, M. Pasanen10, G. Sobrito1 and M. Uemura20

1  Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Torino, Via Osservatorio 20, 10025 Pino Torinese (TO), Italy
2  Abastumani Observatory, 383762 Abastumani, Georgia
3  Astrophysikalisches Institute Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
4  Landessternwarte Heidelberg-Königstuhl, Königstuhl 12, 69117 Heidelberg, Germany
5  Ulugh Beg Astronomical Institute, Academy of Sciences of Uzbekistan, 33 Astronomical Str., Tashkent 700052, Uzbekistan
6  Isaac Newton Institute of Chile, Uzbekistan Branch
7  Physics Department, University of Crete, 710 03 Heraklion, Crete, Greece
8  IESL, Foundation for Research and Technology-Hellas, 711 10 Heraklion, Crete, Greece
9  Astronomical Institute, Osaka Kyoiku University, Kashiwara-shi, Osaka, 582-8582 Japan
10  Tuorla Observatory, 21500 Piikkiö, Finland
11  Osservatorio Astronomico, Università di Perugia, Via B. Bonfigli, 06126 Perugia, Italy
12  Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
13  Department of Physics and Astronomy, University of Victoria, PO Box 3055, Victoria, BC V8W 3P6, Canada
14  Institute for Astrophysical Research, Boston University, 725 Commonwealth Ave., Boston, MA 02215, USA
15  Astronomical Institute, St.-Petersburg State University, Bibliotechnaya Pl. 2, Petrodvoretz, 198504 St.-Petersburg, Russia
16  Isaac Newton Institute of Chile, St.-Petersburg Branch
17  Dipartimento di Fisica, Università La Sapienza, Piazzale A. Moro 2, 00185 Roma, Italy
18  Department of Physics and Astronomy, University of Missouri-St. Louis, 8001 Natural Bridge Road, St. Louis, MO 63121, USA
19  Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
20  Department of Astronomy, Faculty of Science, Kyoto University, Kyoto, Japan
21  Clarke and Coyote Astrophysical Observatory, PO Box 930, Wilton, CA 95693, USA
22  Instituto de Astronomía, UNAM, Apdo. Postal 70-264, 04510 México DF, Mexico
23  Nyrölä Observatory, Jyväskylän Sirius ry, Kyllikinkatu 1, 40950 Jyväskylä, Finland
24  Guadarrama Observatory, C/ San Pablo 5, Villalba 28409, Madrid, Spain
25  Department of Physics, University of Colorado at Denver, PO Box 173364, Denver, CO 80217-3364, USA
26  Department of Physics and Astronomy, Western Kentucky University, 1 Big Red Way, Bowling Green, KY 42104, USA
27  Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30303, USA
28  Center for Astrophysics, Guangzhou University, Guangzhou 510400, China
29  Department of Chemistry, Physics, & Astronomy, Francis Marion University, PO Box 100547, Florence, SC 29501-0547, USA
30  Dipartimento di Fisica Generale, Università di Torino, Via P. Giuria 1, 10125 Torino, Italy

(Received 7 March 2002 / Accepted 29 April 2002)

We present UBVRI light curves of BL Lacertae from May 2000 to January 2001, obtained by 24 telescopes in 11 countries. More than 15 000 observations were performed in that period, which was the extension of the Whole Earth Blazar Telescope (WEBT) campaign originally planned for July-August 2000. The exceptional sampling reached allows one to follow the flux behaviour in fine detail. Two different phases can be distinguished in the light curves: a first, relatively low-brightness phase is followed by an outburst phase, after a more than $1\rm\,mag$ brightening in a few weeks. Both the time duration (about $100\rm\,d$) and the variation amplitude (roughly $0.9\rm\,mag$) are similar in the two phases. Rapid flux oscillations are present all the time, involving variations up to a few tenths of mag on hour time scales, and witnessing an intense intraday activity of this source. In particular, a half-mag brightness decrease in about $7\rm\,h$ was detected on August 8-9, 2000, immediately followed by a ~ $0.4 \rm\, mag$ brightening in $1.7\rm\,h$. Colour indexes have been derived by coupling the highest precision B and R data taken by the same instrument within $20\rm\,min$ and after subtracting the host galaxy contribution from the fluxes. The 620 indexes obtained show that the optical spectrum is weakly sensitive to the long-term trend, while it strictly follows the short-term flux behaviour, becoming bluer when the brightness increases. Thus, spectral changes are not related to the host galaxy contribution, but they are an intrinsic feature of fast flares. We suggest that the achromatic mechanism causing the long-term flux base-level modulation can be envisaged in a variation of the relativistic Doppler beaming factor, and that this variation is likely due to a change of the viewing angle. Discrete correlation function (DCF) analysis reveals the existence of a characteristic time scale of variability of ~ $7\rm\,h$ in the light curve of the core WEBT campaign, while no measurable time delay between variations in the B and R bands is found.

Key words: galaxies: active -- galaxies: BL Lacertae objects: general -- galaxies: BL Lacertae objects: individual: BL Lacertae -- galaxies: jets -- quasars: general

Offprint request: M. Villata, villata@to.astro.it

SIMBAD Objects

© ESO 2002

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